An inverter is an apparatus that is used for producing an alternating voltage with a variable frequency from a direct voltage source. Inverters can operate by switching either a positive or a negative voltage from the DC voltage source to the load. Output switches of the inverter can be modulated according to a control method for producing a desired voltage to the load. A known application of an inverter is in a frequency converter. Frequency converters, in turn, can be used for driving a motor in a controllable fashion.
In some situations, high-powered inverters are produced from parallel connected inverter units. Inverters that are connected in parallel can feed the same load and receive the same instructions for the output switches. Outputs of the parallel connected inverters can be equipped with output reactors and the corresponding phases of parallel inverter units are connected together after the reactors and a single cable or multiple cables per phase are connected to the load. Alternatively, each inverter unit can be connected with its own cabling to the load, and the outputs can be connected in parallel first in the terminals of the load.
Although the parallel operated inverter units can receive simultaneous and similar control signals, the parallel units do not behave similarly. Due to parameter differences of switch components and differing impedances in parallel branches, the currents between the units can be unequal in magnitude. Such a current imbalance can stress the components unevenly and wear switch components with higher current prematurely. A higher current in a switch component can result in a higher dissipated power and, further, a higher temperature of the component.
Current imbalance has been addressed by modifying switch control pulses in order to balance the currents. The control pulses can be modified by delaying a turn-on time instant for a switch that has the highest current or by delaying turn-off time instants for a switch that has the smallest current. One such method is disclosed in EP 0 524 398. In these solutions, the conducting times of the parallel components are modified to equalize stresses to the switch components on the basis of measured inverter unit currents.
The parallel operated switch components can still be unevenly worn even if the currents have been balanced. For example, cooling of the switch components may differ from each other for some reason. The reasons for different coolings include an uneven flow of cooling air in a cabinet, deterioration of the cooling of some inverter modules due to impurities in the cooling air, weak thermal connections between heat sinks and the switch components, and so forth.
The modification of the control pulses for balancing the currents on the basis of current measurement may thus lead to a situation in which the switch component is damaged due to an excessive temperature.